Satellite-based broadcasting systems provide an adequate communication link only in rural areas, in which only a small number of e.g. bridges exist. Additionally, rural areas usually do not have skyscrapers. Skyscrapers as well as bridges or, generally, densly built-up areas are obstacles to satellite-based communication systems, since carrier frequencies used for such communication links involve that a channel between a sender, e.g., a satellite, and a receiver, i. e. a mobile or stationary receiver, is characterised by the line of visual contact (line of sight) between the sender and the receiver. If a skyscraper comes into the line of visual contact, i.e., the transmission channel between the satellite and the receiver, which may be positioned in a car, the received signal power will decrease substantially.
Generally, it can be stated that in wireless systems (radio systems), changes in the physical environment cause the channel to fade. These changes include both relative movement between transmitter and receiver and moving scatters/reflectors in the surrounding space. In theoretical studies of wireless systems, the real channels are usually modelled so that they result in trackable analysis. The two major classes of fading characteristics are known as Rayleigh and Rician. A Rayleigh-fading environment assumes no line of sight and no fixed reflectors/scatters. The expected value of the fading is zero. If there is a line of sight, this can be modelled by Rician-fading, which has the same characteristics as the Rayleigh-fading, except for a non-zero expected radio.
Modern digital broadcasting systems know several means for reducing the impact of a channel fading. These concepts comprise channel coding on the one hand and several kinds of diversity on the other hand. The European standard for digital audio broadcasting (DAB), set out in Radio Broadcasting Systems; Digital Audio Broadcasting (DAB) To Mobile, Portable and Fixed Receivers, ETS 300 401, ETS I--European Telecommunications Standards Institute, Valbonne, France, February 1995, uses differential quadrature phase-shift keying (DQPSK) as modulation technique. The channel encoding process is based on punctured convolutional coding, which allows both equal and unequal error protection. As a mother code, a convolutional code having a code rate of 1/4, a constraint length 7, and octal polynominals is used. The puncturing procedure allows the effective code rate to vary between 8/9 and 1/4. Channel coding by means of punctured convolutional codes is described in "Punctured Convolutional Codes of Rate (n-1)/n and Simplified Maximum Likelihood Decoding", J. Bibb Cain et al., IEEE Transactions on Information Theory, Vol. IT-25, No. 1, January 1979.
Punctured convolutional codes can be used in connection with many modulation techniques, such as OFDM, BPSK, QAM, etc.
Different channel encoding techniques are outlined in "Channel Coding with Multilevel/Phase Signals", Gottfried Ungerboeck, IEEE Transactions on Information Theory, Vol. IT 28, No. 1, pages 55 to 66, January 1982.
Bitstreams encoded by means of a convolutional encoder can be decoded by a decoder, in which the well-known Viterbi algorithm is implemented. This algorithm is capable of using the channel state information (see P. Hoeher "TCM on Frequency-Selective Length-Mobile Fading Channels", Proc. Tirrenia International Workshop Digital Communication, Tirrenia, Italy, September 1991). The Viterbi algorithm can be modified to provide reliability estimates together with the decoded sequence. This enables soft decoding. By applying a soft-output Viterbi algorithm, an improvement of about 2 dB is obtained in comparison to systems that implement "hard" decision.